In recent years, the development of a thin display device such as a liquid crystal display device and an organic EL display device, etc. has been rapidly promoted. In many cases, such a thin display device includes an active matrix substrate on which a switching element is arranged for each one of a plurality of pixels to drive the pixel for the purpose of increasing the visual quality of the display device.
For example, a liquid crystal display device includes an active matrix substrate, an opposing substrate opposed to the active matrix substrate, and a liquid crystal layer provided between the substrates. When the liquid crystal display device performs transparent display, a back light serving as a light device is arranged on an opposite side of the active matrix substrate to a side thereof on which the liquid crystal layer is provided.
As the active matrix substrate, as described, for example, in Patent Document 1, a structure including a pixel electrode formed of an indium tin oxide (ITO) film as a transparent conductive film and a thin-film transistor (TFT) as a switching element connected to the pixel electrode has been generally known. The TFT includes, for example, a semiconductor layer made of poly silicon (p-Si), and has a so-called top-gate type structure.
The active matrix substrate includes a gate insulating film covering the semiconductor layer, an inorganic insulating layer covering the gate insulating film and a gate electrode, and an organic insulating layer stacked on the inorganic insulating layer. On a surface of the organic insulating layer, formed is a pixel electrode. In this structure, the organic insulating layer is provided to prevent short-circuiting between a source electrode formed on a surface of the inorganic insulating layer and the pixel electrode.
Since a plurality of insulating films, i.e., the organic insulating film, the inorganic insulating layer, and the gate insulating film are stacked under the pixel electrode, the stacked films absorb or reflect a part of illuminating light from the back light and, as a result, it is disadvantageously difficult to increase use efficiency of light from the back light.
Also, researches of TFT using a transparent oxide semiconductor film made of, for example, In—Ga—Zn—O as a channel layer have been launched in recent years.
Such oxide semiconductor has high ion binding property, and a difference in electron mobility between crystal and amorphous material is small. Therefore, relatively high electron mobility can be achieved even in an amorphous state. Moreover, there is another advantage of oxide semiconductor that an amorphous film can be formed at room temperature using, for example, sputtering.
For example, in Patent Document 2, as illustrated in FIG. 18 showing an enlarged cross-sectional view of a conventional active matrix substrate 100 including a TFT 101, the active matrix substrate 100 includes a channel portion 114, a source portion 115, a drain portion 116, a pixel electrode 113, a terminal portion 118 of a gate signal line 111, and a terminal portion 117 of a source signal line 112 which are made of the same oxide semiconductor. A gate electrode 110 is arranged to be opposed to the channel portion 114.
In fabricating the active matrix substrate 100, after forming all layer structures on a substrate 102, an opening portion 130 is formed in a protective insulating film 119 as a topmost layer and a gate insulating film 120 provided under the protective insulating film 119 so as to be located at a desired position. Then, an oxide semiconductor film 126 is exposed to reducing plasma or plasma containing a doping element through the opening portion 130, thereby simultaneously reducing resistances of the terminal portions 118 and 117, the source portion 115, the drain portion 116, and the pixel electrode 113.
Thus, by forming the TFT using an oxide semiconductor film, a different structure from the structure in which the plurality of insulating films, i.e., the organic insulating film, the inorganic insulating layer, and the gate insulating film are stacked under the pixel electrode, which has been described before, can be obtained. Therefore, use efficiency of light from a back light can be increased, and also, electron mobility in the TFT can be increased to reduce an off-leakage current.